The survival of dynamical fossils in dwarf spheroidal galaxies in conventional and modified dynamics

TL;DR

This paper examines the longevity of stellar substructures in dwarf spheroidal galaxies under modified gravity theories, finding that such features are unlikely to survive long, challenging current models like MOND and LambdaCDM.

Contribution

It provides the first detailed analysis of the survival of dynamical fossils in UMi under MOND, highlighting challenges for alternative gravity theories.

Findings

Stellar clumps in UMi dissolve within 1.25 Gyr under MOND.

Survival of such clumps is difficult for both MOND and LambdaCDM models.

The observed cold clump in UMi challenges current gravitational models.

Abstract

The survival of unbound density substructure against orbital mixing imposes strong constraints on the slope of the underlying gravitational potential and provides a new test on modified gravities. Here we investigate whether the interpretation that the stellar clump in Ursa Minor (UMi) dwarf spheroidal galaxy is a `dynamical fossil' is consistent with Modified Newtonian dynamics (MOND). For UMi mass models inferred by fitting the velocity dispersion profile, the stellar clump around the second peak of UMi is erased very rapidly, within 1.25 Gyr (6.5 orbits), even with the inclusion of self-gravity. We find that the clump can hardly survive for more than 2 Gyr even under more generous conditions. Alternative scenarios which could lead to a kinematically cold clump are discussed but, so far, none of them were found to be fully satisfactory. Our conclusion is that the cold clump in UMi poses a challenge for both LambdaCDM and MOND.